Conventional apparatus for threshing grain generally includes a threshing unit that has a threshing cylinder operatively associated with the concave member or with a member that may have both a concave section and a grate section. The grain from the concave and grate sections is usually discharged onto a reciprocating chaffer sieve which passes grain and tailings therefrom to a reciprocating grain sieve. A clean grain auger picks up the clean grain for harvest and a tailings auger returns the tailings to the threshing unit. This general type threshing apparatus is disclosed in U.S. Pat. Nos. 3,537,460; 3,833,006 and 3,982,549. In these patents, grain separation is primarily dependent on the action of gravity so as to appreciably limit the capacity and cleaning efficiency of the threshing apparatus.
To more readily get rid of chaff from the threshed material, air under pressure has been utilized in combination with a threshing unit as exemplified by the disclosures of U.S. Pat. Nos. 1,226,865; 1,184,999; 2,053,148 and 2,811,158. In the separation of grain from the threshed material, centrifugal action has been utilized in the threshing mechanisms shown in U.S. Pat. No. 1,887,817, which also shows the use of air for removing chaff. In U.S. Pat. No. 3,425,423 the chaff is removed by an flow and, additionally, a blast of air is utilized with a rotatable separating screen tube to dislodge from the top portion of the inner peripheral surface of the tube any material which does not drop by the action of gravity during rotation of the screen tube.
Although pressurized air for chaff removal, grain separating cylinders that may be stationary or rotatably or reciprocally movable and an air blast acting on separating cylinders have been used in various combinations, the resultant threshing mechanisms have been inherently incapable of operating at a continuous high capacity with a sustained and efficient grain separating action.